1. Field of the Invention
The present invention relates to an inverter control apparatus that controls an inverter. Also the present invention relates to a motor drive system.
2. Description of the Related Art
To perform vector control of a motor by supplying three-phase AC power to the motor, it is necessary to detect phase currents (e.g., a U-phase current and a V-phase current) of two phases of three phases of U, V and W. As a current detection system for detecting phase currents for two phases, the one-shunt current detection system (single-shunt current detection system) has been proposed. In the one-shunt current detection system, a current sensor is disposed on a wiring that connects an inverter and a DC power source and detects phase currents for two phases by sampling output signals from the current sensor at appropriate timings.
An inverter control method useful to a motor drive system that employs the one-shunt current detection system is proposed by the applicant of the present application. In this inverter control method, a voltage command vector for specifying an output voltage vector (an applied voltage vector to a motor) from a PWM inverter is prepared; then, the voltage command vector is corrected on an ab coordinate system that includes an a axis and a b axis as the coordinate axes; thus, a secure detection of a phase current is achieved. The a axis is an axis that rotates in stepwise fashion for every 60 degrees in electrical angle depending on the phase of a voltage command vector on a fixed coordinate system. The b axis is an axis that leads the a axis by an electrical angle of 90 degrees in phase.
On the other hand, in driving a three-phase motor, generally the sine-wave PWM control is used. In the sine-wave PWM control, a sine-wave voltage is supplied to each phase of a motor (see FIG. 4). However, if it is desired to supply a voltage to a motor that exceeds the maximum voltage that is able to be output in the sine-wave PWM control, the rectangular-wave driving is used instead of the sine-wave PWM control. In the rectangular-wave driving, a voltage that has a voltage waveform similar to a rectangular wave is applied to each phase of a motor (see FIG. 7). The modulation system or the PWM to achieve such rectangular-wave driving is generally called overmodulation or overmodulation PWM.
In an inverter control apparatus that uses an ab coordinate system, a technology that introduces overmodulation has not been proposed yet. Because an ab coordinate system has a special characteristic that the ab coordinate system rotates in stepwise fashion for every 60 degrees in electrical angle depending on the phase of a voltage command vector, a modulation technology suitable for the characteristic is necessary.
As inverter control methods for overmodulation, although there are some conventional technologies, they are unrelated to the technology that introduces overmodulation into an ab coordinate system.